1. Field of the Invention
The present invention pertains to a method and apparatus for capturing digital radiographic images. More particularly, the present invention relates to a method and associated apparatus for capturing and readout of electrical charges representing a latent radiographic image in a unique microcapacitor matrix panel to obtain an electrical signal representing a radiogram.
2. Description of the Related Art
Traditional radiography employs a silver halide photosensitive film in a light tight cassette enclosure, to capture a latent radiographic image, which is subsequently rendered visible following chemical development and fixing. Because silver halide film is not very sensitive to X-ray radiation, and large exposures are required to obtain an image, most applications use a combination of an intensifying screen comprising a phosphor layer, with the silver halide film to achieve lower exposures.
Radiograms have also been produced by capturing a latent radiographic image using a photoconductive plate in a xeroradiographic process. In this instance, a photoconductive plate sensitive to X-ray radiation comprising at least a photoconductive layer coated over a conductive backing layer is first charged by passing under a charging station which generates corona ions. Positive or negative charge is uniformly deposited over the plate surface. The plate is next exposed to X-ray radiation. Depending on the intensity of the incident radiation, electron hole pairs generated by the X-ray radiation are separated by a field incident to the charge laid over the surface and move along the field to recombine with the surface charge. After X-ray exposure, a latent image in the form of electrical charges of varying magnitude remain on the plate surface, representing a latent electrostatic radiogram. This latent image may then be rendered visible by toning and preferably transferring onto a receiving surface for better viewing.
More recent developments include the use of an electrostatic image capture element to capture a latent X-ray image, the element comprising a photoconductive layer over a conductive support, the photoconductive layer also covered by a dielectric layer, and the dielectric layer overcoated with a transparent electrode. A biasing voltage is applied between the transparent electrode and the conductive support to charge the element which is a large parallel plate capacitor. While the bias voltage is applied, the element is exposed to image wise modulated X-ray radiation. Following exposure, the bias is removed and a latent image is preserved as a charge distribution stored across the dielectric layer. The problem with this element structure is that the latent image represented by local charge variations is a very small signal charge that must be extracted in the presence of random noise in the total capacitive charge in the full plate. Signal to noise ratio is typically poor.
In an effort to improve the signal to noise ratio, the transparent electrode is laid over the dielectric layer as a plurality of pixel size microplates having an area commensurate with the area of the smallest resolvable element in the image. In this manner, the overall plate capacity is reduced and the signal extracted per picture element has a better signal to noise ratio. Methods to readout the latent image include, inter alia, scanning the length of the transparent electrode with a laser beam while reading the charge flow from each of the microcapacitors formed between the microplates and the conductive plate. While this element is a vast improvement over the continuous electrode structure covering the full plate, the mode of use of this plate is somewhat complex particularly with respect to the manner in which the original charging of the microplates occurs.